The present invention relates to a method of passivating a gas vessel or component of a gas transfer system to reduce corrosion. More particularly, the present invention relates to an improved method of applying a silicon passivation layer to the interior surface of gas storage vessels and components of transfer systems to passivate the interior surface.
The present invention overcomes many known deficiencies of using silicon as a passivation layer for metal (ferrous and non-ferrous) surfaces.
Previous art has focused on layers of silicon modified by oxidation to prevent adsorption. Other previous art has looked at the use of silanes or silicon hydrides passed through metal surfaces at low temperatures to passivate the metal surface. This invention is specifically optimized for storage vessels used for holding species which degrade, are adsorbed or attack metal surfaces (organo-sulfurs, hydrogen sulfide, alcohols, acetates, metal hydries, hydrochloric acid, nitric acid, sulfuric acid).
The prior art has utilized a single treatment of silicon hydride gases, either for silicon deposition or adsorption to metal surfaces, to impart passivation. This invention utilizes singla and multiple treatments with the silicon hydride gases to impart desired passivation by deposition of silicon.
The prior art also has not focused on the surface roughness of articles to be coated with a silicon layer for passivation. This work has noted that the number of applications of silicon layers is dependent on surface roughness.
Prior art also indicates preparation of metals surfaces by exposure to reducing gases prior to silicon deposition. This invention does not utilize such a pretreatment to achieve a passive surface.
The present invention provides a method of passivating any surface of a gas storage and transfer system to protect the surface against corrosion. The present invention also provides gas storage vessels and gas transfer components having corrosive gas contact surfaces which have been passivated in accordance with the method of the present invention.
In the method of the present invention, the surface of, for example, the storage vessel is initially preconditioned by dehydrating the interior surface of the vessel. In the dehydration step, the vessel is heated to a temperature in the range of 360xc2x0 to 600xc2x0 C. for 30 to 240 minutes. The storage vessel is preferably heated in an inert gas or in a vacuum.
After the surfaces of the vessel have been dehydrated, the interior of the vessel is evacuated. A silicon hydride gas is introduced into the storage vessel. The vessel and gas contained therein are heated and pressurized to decompose the silicon, hydride gas in the chamber. As the gas decomposes, a layer of silicon is deposited on the interior surface of the vessel.
The duration of the silicon deposition step is controlled to prevent the formation silicon dust in the vessel. At the end of the silicon deposition step, the vessel is purged with an inert gas to remove the silicon hydride gas. If the silicon layer completely covers the interior surface of the vessel, the vessel is then evacuated and cooled to room temperature. If the silicon layer does not completely cover the interior surface, the silicon deposition step is repeated until the interior surface is completely covered and thereby passivated.
In the method of the present invention, the silicon hydride gas is preferably selected from the group comprising SiH4 and SinHn+2. The silicon hydride gas is heated to a temperature approximately equal to the gas""s decomposition temperature, preferably to a temperature in the range of 360xc2x0 to 600xc2x0 C. Preferably, the silicon hydride gas is pressurized to a pressure in the range of 2 to 45 p.s.i.a.
The duration of the silicon deposition step should be in range of 30 to 240 minutes depending on the roughness average (RA) of the surface. Therefore, the method of the present invention may also include the step of measuring the average surface roughness (RA) of the interior surface of the vessel before dehydrating the vessel. If the surface RA is less than about 20 microinches, the silicon deposition step should be repeated 1 to 2 times or until the silicon layer is 120 to 500 angstroms in thickness. If the surface RA is greater than about 20 microinches, the silicon deposition step should be repeated 2 to 5 times or until the silicon layer is 501 to 50,000 angstroms.
The present invention also provides a corrosion resistant gas storage vessel or gas transfer system component having a passivated surface. The metallic interior surface of the vessel or component has an average surface roughness RA. A silicon layer is formed over the entire interior surface to passivate the surface. The silicon layer is formed from a plurality of layers of silicon and is substantially free of silicon dust.
If the surface has an RA less than about 20 microinches, the silicon layer has a thickness in the range of 120 to 500 angstroms. If the surface has an RA greater than about 20 microinches, the silicon layer has a thickness in the range of 501 to 50,000 angstroms.